Visualization of a light's spatial distribution — beam angle, falloff, hotspot placement. Use it to position fixtures precisely instead of guessing.
You need a field chart when you want to position a light without making fifteen attempts. It shows you at a glance how your light source fills the room — where the most intense hotspot sits, how quickly the intensity drops off, where the edges become blurred. This is planning instead of trial and error.
The chart works simplified like a top view or a cross-section: The light source is at the origin, lines or area gradients show the direction of spread and brightness gradations. Some are polar (semicircle or full circle), others Cartesian (rectangular). For reflectors, spotlights, or specialized lights like Fresnels, the manufacturer often provides such charts. For modern LED spots, this information can be found in the data sheet or in the luminaire configuration software. You read from it: beam angle (how wide does it get?), field angle (where does the brightness drop to 50%?), falloff curve (how quickly does it get dark?), and where exactly the hotspot is located in relation to the geometric center.
On set, you use it like this: You have a difficult lighting situation — let's say, a wall needs to be lit evenly, but it must remain dark behind it. Instead of moving the light and constantly measuring, you use the field chart as a guide. You immediately recognize whether a certain beam angle is sufficient or if you need to stop down, use a snooted reflector, or pan. This saves time and reduces errors. Especially important for depth-of-field shots or when you have to work with very limited space — for example, in a small hotel room.
A common mistake: Beginners think the charts are exact representations of reality. They are not — room reflections, atmospheric haze, surface properties play a role. But as a guide for rough positioning, field charts are indispensable. Modern lighting simulation software (e.g., ETC tools for complex setups) creates these charts for any scenario. For documentary or quick jobs, the standard charts are perfectly sufficient. Think of the field chart as a map — not perfect, but better than fumbling in the dark.